[Mesa-dev] [RFC] i965: alternative to memctx for cleaning up nir variants

Rob Clark robdclark at gmail.com
Mon Dec 28 10:35:08 PST 2015

On Mon, Dec 28, 2015 at 12:37 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
> On Mon, Dec 28, 2015 at 10:13 AM, Rob Clark <robdclark at gmail.com> wrote:
>> On Tue, Dec 22, 2015 at 10:11 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>> On Tue, Dec 22, 2015 at 9:55 PM, Rob Clark <robdclark at gmail.com> wrote:
>>>> On Tue, Dec 22, 2015 at 9:47 PM, Connor Abbott <cwabbott0 at gmail.com> wrote:
>>>>> On Tue, Dec 22, 2015 at 9:02 PM, Rob Clark <robdclark at gmail.com> wrote:
>>>>>> On Mon, Dec 21, 2015 at 1:48 PM, Jason Ekstrand <jason at jlekstrand.net> wrote:
>>>>>>> I think two different concepts of ownership are getting conflated here:
>>>>>>> Right/responsibility to delete and right to modify.
>>>>>>> The way I understand it, gallium, as it stands, gives neither to the driver.
>>>>>>> A back-end using NIR requires the right to modify but who deletes it doesn't
>>>>>>> ultimately matter.  I think it's dangerous to pass one of these rights to
>>>>>>> the driver and not the other but we need to think about both.
>>>>>> yeah, uneasy about driver modifying the IR if the state tracker is
>>>>>> still going to potentially spin off variants of the IR.. that sounds
>>>>>> like madness.
>>>>>> The refcnt'ing I proposed does deal w/ right to modify vs delete via
>>>>>> nir_shader(_is)_mutable() which returns something that is guaranteed
>>>>>> to be safe to modify (ie. has only a single reference)
>>>>>>> What I'm trying to say is that we have two options here:
>>>>>>> 1) gallium passes IR to the back-end that it is free to modify and is
>>>>>>> required to delete when it's done.
>>>>>> with refcnt'ing, s/delete/unref/
>>>>>> The idea is, the st transfers ownership of the reference it passes to
>>>>>> the driver.  If the st wants to hang on to a reference itself, it must
>>>>>> increment the refcnt before passing to the driver (backend).
>>>>>> Without refcnt'ing, I suppose we could (since we don't have to follow
>>>>>> TGSI semantics), just decree that the driver always takes ownership of
>>>>>> the copy passed in, and if the st wants to hang on to a copy too, then
>>>>>> it must clone.  I suppose this would work well enough for
>>>>>> freedreno/vc4, which both end up generating variants later.  It does
>>>>>> force an extra clone for drivers that immediately translate into their
>>>>>> own backend IR and don't need to keep the NIR around, for example.
>>>>>> Maybe that is not worth caring about (since at this point it is
>>>>>> hypothetical).
>>>>> While always cloning does have this disadvantage, I don't think it's
>>>>> really relevant here. Even if the driver throws away the NIR
>>>>> immediately after consuming it, almost invariably it's going to want
>>>>> to modify  it. The generic NIR passed in by the state tracker (other
>>>>> IR -> NIR + some optimizations) is almost never going to be the same
>>>>> as the NIR after going through driver-specific lowering passes, which
>>>>> means that drivers are never going to want a read-only version of the
>>>>> IR. In light of that, I think making the driver own the IR passed in
>>>>> seems like the most sensible thing.
>>>> well, unless the driver is already doing it's own lowering in it's own
>>>> native IR..
>>> Well, if you're not doing any lowering in NIR, then you aren't really
>>> taking any advantage of it. I can't see a plausible scenario where all
>>> the lowering is done in the driver's own IR -- and as soon as you do
>>> anything in NIR, you need the driver-owns-IR semantics.
>> When it comes to shader variants, I have a mix, with some things
>> lowered in nir and others just handled in backend..
>> The re-work / cleanup that I have had on a branch for a while now
>> (since it is currently blocked on refcnt'ing) does a first round of
>> variant-key independent NIR lowering/opt passes.  And then at draw
>> time, if the variant key has anything that is lowered in nir, I do a
>> second round.
>>>> Maybe it is too much of a hypothetical.. I still think refcnt'ing
>>>> gives some nice flexibility to deal with various scenarios, and having
>>>> to call nir_shader_unref() isn't so much of a burden.
>>> Still, I can't see how this flexibility is at all useful, and it seems
>>> like overkill since the driver will always want a mutable version of
>>> the IR anyways.
>> Well, due to the structure I mentioned above, at draw time when I need
>> to generate a variant with nothing lowered in NIR, I simply incr the
>> refcnt on the IR which has already gone through first round of NIR
>> passes, and pass that in to my back end compiler.  At the end, once
>> the shader binary is generated, I can unconditionally unref the
>> nir_shader without having to care.
>> Without refcnt'ing I'd either have to generate a pointless clone or
>> keep track that the nir_shader should not actually be free'd.  Not
>> impossible, just a bit more ugly.
> Assuming you do all your variant management in your driver's IR, then
> you don't need to do anything. If you do some variant management in
> NIR, then in the function where you do the NIR-based lowering you can
> check if you need to do any lowering based on the shader key, clone
> first, and give the NIR->ir3 function the cloned IR and then free it.
> It might be a "bit more ugly," but it's really not that much different
> from the refcounting, and when the extra shader gets created/freed is
> made explicit.

I'd have to re-arrange things some compared to how the backend works
now..  not impossible, but annoying.  Also, I could more easily unref
the NIR once I've converted into ir3 rather than waiting until after
the backend passes.

>> (The gallium glsl_to_nir stuff is also currently using refcnt'ing,
>> although at least for freedreno/ir3 it isn't strictly needed.. I could
>> just unconditionally clone in the state tracker.  That said, I'm still
>> of the opinion that refcnt'ing could be useful to some other driver
>> someday)
> "It could be useful to some driver someday" isn't a good argument for
> adding stuff today. We've already had enough examples of things in NIR
> that we added because we thought it was useful, but turned out not to
> be.

well, it is useful to one driver today, as I explained..  so it is
more a matter of "It could be also useful to some other driver...".
Otherwise I would agree with you.


>> BR,
>> -R
>>>> BR,
>>>> -R
>>>>>> (I guess nouveau is the one driver, that if it ever consumed NIR,
>>>>>> would translate immediately into it's own backend IR?)
>>>>>>> 2) gallium passes read-only IR to the back-end and it always makes a copy.
>>>>>> This is how it is w/ TGSI, but I think with NIR we are free to make a
>>>>>> clean break.  And we *definitely* want to avoid
>>>>>> the-driver-always-copies semantics..
>>>>>>> It sounds like, from what Marek is saying, that gallium is currently doing
>>>>>>> (2) and changing it to (1) would be painful.  I think reference counting is
>>>>>>> more like an awkward option 1.5 than option 3.  Reference counting would
>>>>>>> mean that gallium passes a reference to the driver which it is expected to
>>>>>>> unref but may keep a second reference if it wants to keep the driver from
>>>>>>> modifying it.  Then the driver may or may not make a copy based on the
>>>>>>> number of references.  Why don't we just make it explicit and add a
>>>>>>> read-only bit and call it a day.
>>>>>>> One of the reasons I don't like passing a reference is that it effectively
>>>>>>> puts allocation and freeing in different components of the driver.
>>>>>> With refcnt'ing you should talk in terms of ref/unref rather than
>>>>>> allocate/free.. imho.  Although maybe that is what you meant.  (In
>>>>>> which case, yes, that was my idea, that passing in to driver transfers
>>>>>> ownership of the passed reference.)
>>>>>>> This
>>>>>>> means that if and driver doesn't care at all about the shader that gets
>>>>>>> passed in, it still has to under it to avoid a memory leak.  You can't have
>>>>>>> the driver take the reference because then, either it comes in with a
>>>>>>> recount of 0 and should have been deleted, or the "can I modify this" check
>>>>>>> becomes "recount <= 2" which makes no sense.
>>>>>> hmm, no, if ownership of the reference is transferred to the driver,
>>>>>> then it becomes "refcount == 1" (and refcount == 0 should be an
>>>>>> assert, because something has gone horribly wrong)
>>>>>> BR,
>>>>>> -R

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